Unlike most lesser mass murders, the Holocaust didn't come from extensive simmering social (and only secondarily "ethnic") tensions. Instead the Nazi genocide was built on weird, largely faked-up racial
"theories." Not even the most rabid antisemites before the Nazis openly advocated the slaughter of all of Europe's Jewish men, women, and children. The earlier maniacs, like American black-haters, just wanted them out of the country, or kept "in their place."

And as others have noted, the genocidal horror was not created in some benighted backwards tribal society but in one of the most intelectually sophisticated and best educated nations in earth.

It's pointless to compare holocausts. Stalin and Mao each probably killed more people than Hitler (though the 30 million other Europeans who died as a result of Hitler's invasion should not be left off his rap sheet). Auschwitz is an appropriate symbol for all of them, because it was the Nazis, after all, who invented the very idea of a "death camp."

(At least the Aztecs and Mayas, also mass slaughterers of the innocent, truly believed that was the only way to keep the sun burning. The Nazis knew better.)